The art of printing images with inkjet technology is relatively well known. In general, an image is produced by emitting ink drops from a printhead at precise moments so they impact a print medium at a desired location. In a scanning-head embodiment, the printhead is supported by a movable print carriage within a device, such as an inkjet printer, and is caused to reciprocate relative to an advancing print medium. It emits ink at times pursuant to commands of a microprocessor or other controller. The timing of the emissions corresponds to a pattern of pixels of the image being printed. Other than printers, familiar devices incorporating inkjet technology include fax machines, all-in-ones, photo printers, and graphics plotters, to name a few.
Conventionally, a thermal inkjet printhead includes access to a local or remote supply of color or black ink, a heater chip, a nozzle plate attached to or integrated with the heater chip, and an input/output connector, such as a tape automated bond (TAB) circuit, for electrically connecting the heater chip to the printer during use. The heater chip, in turn, typically includes a plurality of thin film resistors (also referred to as “heaters”) fabricated by deposition, patterning and etching on a substrate such as silicon. One or more ink vias cut or etched through a thickness of the substrate serve to fluidly connect the supply of ink to the individual heaters.
Heretofore, conventional heater chip thin films included a relatively thick silicon nitride (SiN) and/or silicon carbide (SiC) layer(s) overlying a resistor layer for reasons relating to passivation. In turn, a cavitation layer lied over the two passivation layers to protect the heater from corrosive ink and bubble collapse occurring in the ink chamber. However, as layers continued to become thinner and more energy efficient over time, thinner passivation seemed unable to provide adequate ESD protection. It some instances, the passivation has been so thin that ESD events damage the resistor layer making it altogether inoperable.
Accordingly, the inkjet printhead arts desire ESD protection despite a continuing trend toward thinner chip configurations.
Appreciating some advances in ESD protection have occurred over time, some prior art products use a serpentine resistive structure at a terminal end of the cavitation layer, for example, to dissipate current of ESD events. However, a disparity exists between heaters closest to the serpentine structure and those farther away. As expected, the closest ones are afforded better ESD protection than the farther ones.
Appreciating ESD events can occur during use, handling and/or manufacturing, other prior art devices contemplate ESD structures for each of the various phases. Namely, some prior art teachings use fuses separating active from inactive components during manufacturing and teach using other structures during use. However, these approaches add undesirable complexity. For example, using devices that are not practically reset able, like conventional fuses and/or preferred breakdown locations, can obviously limit the functionality of such circuits (once triggered, the device cannot be readily reset). Accordingly, alternative circuits might be provided for to address scenarios that might arise after the aforementioned devices are triggered, leading to more complexity.
Further, because ESD protection is often implemented by a single element, such as the serpentine resistive structure or fuse, very little, if any, robustness can be obtained. That is, ESD current dissipation is often limited to a few milliamps. However, many actual ESD events surpass this minimal current dissipation criterion and chips touting ESD protection are routinely destroyed by ESD.
Accordingly, the present inventors have determined that the inkjet printhead arts desire improvements in ESD protection that afford common or similar protection for an entire chip and not for a few select actuators closest to the ESD protection structure. Protection should also be made available for a chip at all times, including use, handling and manufacture, and should be simple in implementation. ESD current dissipation should also contemplate amperage well above the milliamp range. Naturally, any improvements in ESD protection should further contemplate good engineering practices, such as relative inexpensiveness, low power consumption, ease of manufacturing, etc.